1. Field of the Invention
The present invention relates generally to systems and methods for non-destructively detecting material abnormalities beneath a surface, and in particular, to systems and methods for detecting corrosion and cracks in metal surfaces beneath a layer of paint using a terahertz (THz) imaging technique.
2. Description of the Related Art
Paint or other coatings are typically applied to surfaces to protect the surfaces against corrosion or other damage. In some instances, corrosion, cracking, or other damage (material abnormality) begins under the paint or coating and is undetectable by visual inspection. In addition, the paint itself may crack, while the underlying surface is perfectly fine. This may cause unnecessary repairs. Conventionally, inspections may be carried out by one or more of the following non-destructive imaging (NDI) techniques.
Ultrasonic (pulse echo or through transmission) methods can monitor larger defects of, for example, aircraft structures for a whole field, but are not preferred for detection of early or surface deterioration. The techniques are particularly conducive to rapid imaging of a surface and include magneto-optic eddy current imaging, active thermography, optically aided visual inspection, and spectral imaging.
Magneto-optic imaging (MOI) can image corrosion and cracks over a small area the size of the magneto-optic crystal plate used in a hand-held scanner. However, MOI's sensitivity to top-surface corrosion depends on the degree to which the eddy currents are altered and gives rise to anomalies in the induced magnetic field at the surface. It has not been shown conclusively that the MOI technique can detect incipient corrosion that has not yet produced a significant increase in macroscopic surface roughness.
Active Thermography (AT) is an increasingly important technique for detecting subsurface flaws such as delamination, debonding, and second-surface corrosion. AT has a lower sensitivity to incipient corrosion under paint, however, because (1) the initial stages of corrosion do not significantly increase the thermal impedance of the surface compared to a layer of paint alone, and (2) the detailed resolution of incipient corrosion effects at the top surface requires a very high speed infrared camera to resolve surface transients which may appear only in the few milliseconds after the initial flash lamp illumination. In addition, such equipment is prohibitively expensive for use on a wide scale. However, for significant surface corrosion, AT and MOI have the potential to image significant surface corrosion damage and distinguish it from subsurface effects by employing commercially available instrumentation.
Visual Inspection (VI) is used to determine the extent of corrosion damage on a skin of a surface and around fasteners, for example, after the paint or coating has been stripped. As a nondestructive technique for painted aircraft, visual techniques are not amenable to detection of chemical changes or micro-roughness at the paint/metal interface, unless significant corrosion products penetrate through the thickness of the paint.
Spectral Imaging (SI) techniques using near-infrared (NIR) and mid-infrared (MIR) light generally use a compact multi-spectral imaging sensor. This method is based on the partial transparency of many aircraft paints to specific bands of infrared radiation. Using this method, it is possible to detect changes in the chemistry of the metal surface or the primer by analyzing the amplitude of reflected and emitted radiation at specific wavelengths. The layered depth information, i.e., the state of the surface at different depths below the paint, cannot be deduced from a simple SI approach, however.
Photonic techniques offer a potentially rapid, noninvasive and easy approach to detect corrosion and cracks in situ. Art preservationists use Near-IR (NIR) charge-coupled device (CCD) cameras (λ<2 μm) to view paintings that have underdrawings not visible with the human eye. Novel methods of a second-harmonic generation (SHG) imaging, and spectral polarization optical imaging have been proposed to detect the early stages of corrosion under thin layers of paint using NIR. One such example of this technique can be found in U.S. Pat. No. 6,495,833, entitled “Sub-surface Imaging Under Paints and Coatings Using Early Light Spectroscopy,” which issued to Alfano et al. on Dec. 17, 2002 (hereinafter referred to as “Alfano”).
Alfano discloses near-infrared (NIR) optical imaging systems and methods to non-destructively image (NDI) deteriorations or defects in painted metals and artwork beneath painted surfaces. Specfically, back-scattered light is used to determine suitability to monitor corrosion and cracking in metal beneath paints, up to a thickness of about 500 μm. That is, NIR light, which is in the paint transmission zone spanning from 800 nm to 2,000 nm, can be used to assess the quality of metallic structures below the paint level for incipient and advanced stages of corrosion and cracking. NIR light scattered from paint, corrosion, air voids, and metal can be spatially imaged in micrometer sliced sub-surface layers. In addition, spectral, temporal, spatial, nonlinear optical, and polarization gates are employed to distinguish phantoms in turbid media, such as painted corroded metal and cracked specimens, e.g., painted surfaces from airplanes, submarines, ships, automobiles, bridges, etc.
However, the systems and methods disclosed in Alfano, which utilize NIR/MIR optical imaging, also require a gating system in order to create the detected images. This gating system tends to create a more complex and costly imaging system. In addition, this method is not capable of detecting the early stages of corrosion under surface thick paint with thickness larger than 40 μm.
In U.S. patent application Ser. No. 10/455,662, entitled “Systems and Methods for Non-Destructively Detecting Material Abnormalities Beneath a Coated Surface”, which was also filed by Alfano et al., (hereinafter referred to as “Alfano II”), systems and methods are disclosed for detecting the early stages of corrosion under surface thick paint with thickness larger than 40 μm. In Alfano II, a mid-infrared (MIR) 2 μm to 8 μm detection unit illuminates an area of a coated surface and detects light reflected from the illuminated area of the coated surface, and a processing unit for produces an image from optical characteristics received from the MIR detection unit. However, in Alfano II, the processing of the optical characteristics requires extra time and resources, which adds to the complexity and cost associated with those systems and methods.
Therefore, in U.S. patent application Ser. No. 10/653,473, entitled “Systems and Methods for Non-Destructively Detecting Material Abnormalities Beneath a Coated Surface”, which was also filed by Alfano et al., (hereinafter referred to as “Alfano III”), improved systems and methods are disclosed for detecting the early stages of corrosion under surface thick paint with thickness larger than 40 μm. In Alfano III, a mid-infrared (MIR) illumination unit for illuminates an area of the coated surface, and an MIR optical 2-D imager for images the illuminated area of the coated surface.
However, zinc rich primer and metallic based paints and coatings are commonly used on bridges, airplanes, boats and ships. Further, due to strong absorption and small depth of penetration in these kinds of paints or primers, for visible, NIR, and MIR radiations, it is impossible to obtain a clear image for these samples with metallic paint, especially coated by zinc rich primer. Accordingly, a need exists for an improved system and method for inspecting surfaces through a coating, such as zinc rich primer and metallic based paints. A further need exists for such a system and method, which improves upon existing techniques.